In some areas of the world there are really large deposits of very viscous or heavy crude oils or tar sands which are located near the surface of the earth. The overburden in such areas may be as little a two to three hundred feet. In such cases the overburden is thick enough to effectively prevent the surface strip mining of the heavy oil deposit, but it may be, like the heavy oil sand itself, loosely consolidated and not, in effect, a good sealant of the heavy oil sand from the surface.
One especially effective technique used in the past for producing such heavy tar or oil sand formations has been steam flooding of the formation. In steam flooding a pattern of wells is drilled vertically through the overburden and into the heavy oil sand, usually penetrating the entire depth of the sand. Casing is put in place and perforated in the producing interval and then steam generated at the surface is pumped under relatively high pressure down the casing and into the heavy oil formation. In some instances the steam may be pumped for awhile into all of the wells drilled into the producing formation and, after the heat has been used to lower the viscosity of the heavy oil near the wellbore then the steam is removed and the heated, lowered viscosity, oil is pumped to surface, having entered the casing through the perforations. When the heat has dissipated and the heavy oil production falls off, the production is closed and the steam flood resumed. Where the same wells are used to inject steam for awhile and then for production, this technique has been known as the huff and puff method or the push-pull method.
In other instances, some of the vertical wells penetrating the heavy oil sand are used to continuously inject steam while others are used to continuously produce lower viscosity oil heated by the steam. Again, when heavy oil production falls off due to lack of heat, the role of the injectors and producers can be reversed to allow injected steam to reach new portions of the reservoir and the process repeated.
In all of these production techniques, the steam flood is performed at a relatively high pressure (hundreds to over one thousand pounds per square inch or PSI) so as to allow it to penetrate as deeply into the production zone as possible. This can lead to severe environmental problems if the overburden is relatively incompetent which could be penetrated by the heated crude oil, water or steam pumped into the injection wells. Steam breakout to the surface has occurred on more than one occasion in the past when using such production techniques. This can result in serious environmental contamination, particularly if the steam breakout occurs as geysers of hot water and lowered viscosity crude oil onto the earth's surface, as has occurred in the past.
Accordingly, it would be very desirable to develop techniques for effectively producing heavy oils in shallow reservoirs in which steam flooding could be used, but with lower injection pressures so as to avoid surface breakout. The present invention provides production techniques for use in relatively thin or relatively thick heavy oil sand production zones located at shallow depths. The techniques of the present invention are both safe and efficient in recovering the heavy oil from shallow formations and in lowering the required steamflood pressures so as to minimize the potential of surface steam breakthrough.